Do ship strikes threaten the recovery of endangered eastern North Pacific blue whales?

Abstract

Blue whales were targeted in the North Pacific from and are listed as endangered by the IUCN. Despite decades without whaling, abundance estimates for eastern North Pacific (ENP) blue whales (Balaenoptera musculus) suggest little evidence for a recent increase. One possible reason is fatal strikes by large ships, which have affected populations of other cetaceans and resulted in mitigation. We used a population dynamics model to assess the trends and status of ENP blue whales, and the effects of ship strikes. We estimate the population likely never dropped below 460 individuals, and is at 97% of carrying capacity (95% interval 62%-99%). These results suggest density dependence, not ship strikes, is the key reason for the observed lack of increase. We also estimate future strikes will likely have a minimal impact; for example, an 11-fold increase in vessels would lead to a 50% chance the long-term population would be considered depleted. Although we estimate ship strike mitigation would have minimal impacts on population trends and status, current levels of ship strikes are likely above legal limits set by the U.S. The recovery of ENP blue whales from whaling demonstrates the ability of blue whale populations to rebuild under careful management.

Simultaneous tracking of blue whales and large ships demonstrates limited behavioral responses for avoiding collision

“…6). Although recent evidence suggests shipstrikes are not an immediate threat to the survival of this population of blue whales (Monnahan et al 2014), the levels of mortality from ship-strikes are still above legal limits set by the USA. The limited effectiveness of re sponse dives and lack of evidence for lateral movement in reaction to ap proaching ships shows that blue whales are vulnerable to collisions with ships, especially in regions of high ship traffic.…”

“…Collisions between ships and whales are reported regularly throughout the world's oceans and for some species of endangered whales, ship strikes are a major threat to their survival and recovery (Clapham et al 1999, Laist et al 2001, Douglas et al 2008, Berman-Kowalewski et al 2010, Neilson et al 2012, though some recent work has questioned the degree of risk they pose at a population level (Monnahan et al 2014). Often these collisions result in serious injury or death; however, it is highly likely that only a small fraction of struck carcasses are recovered, and reported numbers are most likely underestimates (Williams et al 2011).…”

“…ENP blue whale population estimates are around 2000 to 3000 animals (Calambokidis & Barlow 2004). A recent study suggested that density dependence is likely the key reason for the observed lack of increase (Monnahan et al 2014). The only known source of juvenile and adult mortality for ENP blue whales is fatal collisions between ships and whales (Carretta et al 2012), although noise, chemical pollution, and fishery interactions likely also impact the population.…”

Factors affecting whale detection from large ships in Alaska with implications for whale avoidance

“…Some whale populations are in recovery (e.g. Patenaude et al 2007) or potentially fully recovered from pre-whaling abundances (Monnahan et al 2015), and thus may be able to sustain these added mortality events. Others remain at reduced abundance levels where even small increases in anthropogeniccaused mortality threatens population persistence (Kraus et al 2005, LeDuc et al 2012.…”

Assessing the risk of chronic shipping noise to baleen whales off Southern California, USA

“…Blue whales and fin whales are currently listed as endangered under the US Endangered Species Act (ESA) 1973 and as 'depleted' and 'strategic' under the US Marine Mammal Protection Act (MMPA) 1972, as is a segment of the humpback whale population that feeds off the US West Coast. Although populations of fin whales along the California coast have been in creasing since at least 1991 (Moore & Barlow 2011) and Monnahan et al (2015) suggest that blue whales may have reached carrying capacity, all 3 species still face threats from ship strikes, entanglements, and anthropogenic noise. While poorly understood, use of sound by baleen whales is assumed to include, but not be limited to, hearing conspecific calls.…”